SECT. 3] 



SHALLOW-WATER CARBONATE SEDIMENTS 



571 



of water and the relative abundance of sediment particles derived from coral, 

 coralline algae and Halimeda. The seaward marginal zones are characterized by 

 coral and coralline algae, and the lagoonal or back reef areas by Halimeda. 

 The zone oi Halimeda is best explained in terms of relative rates of production, 

 for we know that living Halimeda is present in abundance in the marginal zone 

 as well as in the lagoonal areas. In the marginal zones, however, the production 

 of sediment by Halimeda is "diluted" by the considerable contribution of coral 

 and coralline algae. Nevertheless, even the relative percentage of Halimeda in 

 the marginal zone is at times as high as it is in the lagoonal zone (Fig. 11). 



REEF INFLUENCE - FUNAFUTI ATOLL 



(DATA FROM HINDE, ROYAL SOCIETY, 19041 



Fl/NAFUri ISIANO 



LAGOON BORING- 



- Approximately IVz miles - 



•-MAIN BORING 



RNG 

 December 1957 



EslimoTed percent 

 HALIMEDA 



100 

 120' 

 140' 

 160' 

 180' 

 200' 

 220' 

 240' 



50 100% 



^ o » 



O^ TD T3 



I ^ y 

 OOP 

 00 "5 



■O X > 

 <u o t: 



0) o. ,o 



Slight 

 percentoqe' 



i -o a 



■ — (l> o 



o _ .6 



<->£ -5 



-S! t 



20% Corals 

 ■ 80% Unconsolidated 

 skeletal debris 



First Colcite-cemented 



skeletol debris 



Procticolly oil aragonite gone 

 First corol casts ond moulds 



^ Calcite- cemented skeletal 

 debris, Corals rare 



Fig. 12. Cross-section, Funafuti Atoll. (Data from Judd, 1904; and Hinde, 1904.) 



The contribution of coral and coralline algae is, therefore, "excess" sediment 

 production over the basic lagoonal percentage of Halimeda. 



The examples above from Florida, Cuba and the Pacific suggest that Hali- 

 meda will predominate in the sediments of those areas where the intensity of 

 water movements and circulation is intermediate between wave-agitated shelf 

 margins and restricted bays. Fig. 12 illustrates how this relationship between 

 abundant Halimeda and lagoonal or bank water circulation may help us to 

 interpret the recent history of an atoll. In Fig. 12 the rather abrupt increase in 

 the percentage of Halimeda in the lagoon sediments at a depth of 160 ft is 

 believed to be a reflection of reef development. Without the reef barrier there 



